Techniques which use embedded optical fibers in composite structures are considered to be very promising for nondestructive detection of damage. In addition to measuring response of composite structures to external stimuli, embedded fiber optic sensors would make ideal “nerves” for sensing the local integrity of such structures. Their optical properties and their compatibility with the properties of composite materials (extremely light weight, small diameter, resistance to corrosion and fatigue, mechanical properties similar to those of composites and insensitivity to ambient magnetic fields) have been exploited in many studies as described and discussed by T. Valis in his publication: Localized and distributed fiber-optic sensors embedded in composite materials, University of Toronto Institute for Aerospace Studies (UTIAS) report 346, September 1992. CN ISSN 0082-5255.
In many cases fiber optic strain sensors have already replaced conventional electrical sensors to measure the excited frequencies and amplitudes in a structure, that were formerly measured with the aid of accelerometers or strain gauges.
Examples of such strain measurements are given by: L. J. Buckley and G. C. Neumeister, Fiber optic strain measurements using an optically-active polymer, Smart Materials Structures, 1(1):1-4, March 1992. Additional examples are given in an article by P. J. Masalkar et al. in pages 230-235 of volume 2443 of the publication of the International Society for Optical Engineering (SPIE): Smart Structures and Integrated Systems 1995, Smart Structures and Materials under the title: Use of optical fiber strain sensor for damage detection in composite structures, published in San Diego, Calif. in February-March 1995. In the same publications in pages 308-312, J. P. Andrews and E. J. Zisk, published an article: Use of optical fiber strain sensor for damage detection in composite structures.
In numerous studies optic fibers have been applied for “health” monitoring systems in composite materials. An example of such use is given by R. M. Measures in a summary of the May 1990 meeting of the Canadian Aeronautics and Space Institute: Progress in the development of fiber-optic smart structures. An additional example is given in an article by D. W. Glossop et al. in the journal Composites, 21(1):71-80, January 1990: Optical fibre damage detection for an aircraft composite leading edge.
Methods for using optic fibers for damage detection include:    1) Use of optic fibers as integral strain sensors and or integral sensors for vibration measurements, either intensity-based or, more commonly, interferometry-based. The latter includes the Bragg, Mach-Zender, Michelson, Fabry-Perot and Sagnac interferometers and high birefrigence polarisation-mode interferometer.    2) Methods employing the fracture of the optic fiber sensor are based on the fact that preliminarily weakened fibers, embedded in a composite structure, crack at points where damage occurs in the structure. These methods use various techniques to determine the location of the crack in the fiber, such as segments of optic fibers connected by Bragg gratings, optical fibers disposed orthogonally, measurements of back reflection and backscattering from the crack. Using preliminarily weakened optic fibers embedded in a composite structure, Measures and Glossop identified cracks in the areas at which damage occurred in the structure, by locating cracks in the optic fibers by light leakage from these cracks through translucent composite material. They successfully located both impact and quasi-statically induced damage, and could map the growth of a region of damage with increasing load.
In contrast to the light emission methods, the approach of the system and method of the present invention identifies cracks in optic fibers by the rise in temperature in the neighborhood of a crack caused by the partial transformation of the light passing through optical fibers to thermal energy.